184 
FOUNDATIONS OF BOTANY 
movements are due to unequal growth induced in the 
younger portions of the plant by the action of gravi- 
Fie. 127. — Stinging 
Hair of Nettle, with 
Nucleus. (Much 
magnified.) The ar- 
rows show the direc- 
tion of the currents 
in the protoplasm. 
tation upon it. Other movements (of 
ordinary foliage leaves, of the floral leaves 
of many flowers, and of other parts of a 
few flowers) are produced by changes in 
the distention or turgescenee of some of the 
cells in the organs which move and have 
nothing to do with growth. The closing 
of the leaves of insect-catching plants is 
briefly described in Sect. 410, and the 
“sleep” of leaves, due to movements of 
the pulvini, was described in Sect. 152. 
A few facts in regard to the opening and 
closing of flowers will be found in 
Sect. 440. 
The stimuli which cause movements of 
leaves or of the irritable parts of flowers 
are of several kinds. Light is the main 
cause which induces leaves to open from 
their night position to that usual in the 
daytime. In the case of flowers, it is 
sometimes light and sometimes warmth 
which causes them to open. Leaves which 
catch insects may be made to close by 
touching them, but the sensitive-plants, 
of which there are several kinds found in 
the United States, and a much more sensi- 
tive one in tropical America, all fold their leaflets, on 
being touched, 
at night. 
into the same position which they assume 
